A vehicle, such as an aircraft, contains many components adhered to one another by adhesives and/or fasteners. Adhesives and fasteners must withstand chemical, thermal, and physical conditions experienced by the vehicle. Adhesives offer greater performance, better design efficiency, and lower weight as compared to fasteners used for connecting the vehicle/aircraft components to one another. In particular, thermosets chemically or physically join vehicle/aircraft components by co-cure or co-bonding processes. However, co-curing involves expensive, precision tooling to properly locate and maintain the vehicle components as well as control of the thermoset resin distribution throughout the curing process. Additionally, co-cured structures typically suffer thickness control issues and ply waviness due to undesirable viscosity of an adhesive. Alternatively, co-bonding involves less extensive tooling but adds the costly labor of surface preparation for the surfaces of the vehicle components to be joined.
Regarding thermoset materials, thermosets have components that are typically fabricated with a peel ply on the surfaces to be joined, which is removed prior to joining, coupled with a surface preparation process such as grit blasting, plasma etching, and or hand sanding of the surfaces to be joined, and followed by bonding of those surfaces with an adhesive. Adhesives are typically thermosets or thermoplastics, such as poly ether ketone ketone (PEKK) or poly ether ether ketone (PEEK). However, thermoplastics are inherently more difficult for adhesive bonding applications in comparison to thermosets due to the chemical nature of poly aryl ether ketone matrix, and their associated processing temperatures when used in structural applications.
Low cost thermoplastics for film joining (adhesion) processes for vehicle components typically employ a melting/fused joining rather than a curing adhesive system. Ideal processing parameters for such a thermoplastic would include:                about 355° C.-385° C. maximum processing temperature for thermoplastic co-consolidation without polymer degradation        Glass transition temperature below about 190° C. for joining thermoset applications without degrading the thermoset component(s)        Polymer that is at least partially amorphous so that crystal formations do not inhibit molecular diffusion at a film to film interface        Polymer should have environmental/chemical resistance equal to or better than vehicle base structures (i.e., vehicle components and, if present, other layers on the components)        Polymer molecular weight should be balanced to provide good mechanical properties while providing rheological properties that promote chain mobility close to the glass transition temperature        Polymer should have adequate adhesion ability        
Existing adhesives do not possess ideal properties. For example, existing materials do not have a low enough processing temperature for suitable use with thermoset materials or the materials degrade at the processing temperature of thermoplastic composites. Polyether ether ketone (PEEK) polymers, for example, have high strength, but processing temperatures for PEEK polymers are high (generally above 355° C.). Furthermore, PEEK polymers are expensive and do not possess sufficiently high Tg values for all applications.
There is a need in the art for polymers that may be used in thermoplastic prepregs/thermosets and/or as an adhesive in vehicle/aircraft structures.